Quantum Interference Effects in Precision Spectroscopy of Helium Fine Structure
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Precision spectroscopy of atomic helium fine structure provides a means of testing fundamental few-body theory as well as determining the fine-structure constant, which characterizes electromagnetic interactions in nature. Progress in both experiment and theory has led to increasingly precise values for the splittings between the n=2 triplet P states of helium, and at the current level of uncertainty, quantum interference can be a significant source of systematic error in measurements of the fine-structure intervals. This work deals with these quantum interference effects, which are due to coherent excitation of the atom to multiple neighbouring states, and may result in substantial shifts in the measured positions of resonance line centres, even if the transition frequencies of adjacent resonances are thousands of natural widths away. The scale of the shifts depends on the measurement technique and the experimental parameters, and therefore a selection of the most precise measurements of the helium fine-structure intervals are analyzed in order to calculate the relevant interference effects. The inclusion of these interference shifts leads to greater consistency between values obtained by several different experimental techniques, and furthers the program of obtaining a high-precision value of the fine-structure constant by comparison between experiment and theory.